Radar sensors are used, for example, in motor vehicles for detecting the surroundings of the vehicle and for locating and determining the relative velocity of vehicles traveling ahead or of oncoming vehicles. They may be used as independent distance warning systems or as part of a driver assistance system, for example, for distance-based adaptive cruise control (ACC).
Precipitation in the form of rain or spray from the road is capable of reflecting and absorbing a portion of the emitted transmission signal and thereby causing a reduction in the range of the radar radiation and thus a reduction in the range within which objects may be reliably located. For reasons of traffic safety, it is important for such a restricted function of the radar sensor to be detectable as early and as reliably as possible.
German Patent Application No. DE 199 45 268 A1 describes a method for detecting a loss of sensitivity (“blinding”) of a radar sensor, e.g., by precipitation, a plurality of parameters being checked on the basis of various criteria. Parameters checked and evaluated on the basis of the criteria are differentiated to various extents and weighted, if necessary. One of the criteria is based on an assessment of the average power of the signals received by the radar sensor. One disadvantage of this method, however, is that the average power depends not only on the presence of diffuse loss sources such as precipitation but also on a plurality of other factors, including specific properties of the radar sensor, assembly tolerances during installation in a motor vehicle as well as temperature and aging influences.
German Patent Application No. DE 10 2006 054 320 A1 describes a method for detecting precipitation using a radar sensor, likewise based on the analysis of a power feature of a received radar signal. This method is suitable for multibeam radar sensors, in particular FMCW (frequency-modulated continuous wave) radar sensors. In this method, the received radar signals of multiple radar beams are each integrated separately and the resulting integrals are compared with one another.
Using radar sensors, the velocity of an object may be determined on the basis of a frequency shift between the emitted radar signal and that reflected by an object and received based on the Doppler effect. To obtain information about the distance of the object from the radar sensor at the same time, information about the transit time of the radar signals is additionally required. In the FMCW radar method, such transit time information is obtainable by subjecting the frequency of the emitted radar signal to frequency modulation using a (often linearly) changing frequency (frequency ramp).
The received radar signal is usually mixed with a portion of the emitted signal to obtain an intermediate frequency signal. The frequency spectrum of the intermediate frequency signal is typically analyzed with the help of a fast Fourier transform (FFT). An object detected by the radar system is reflected in the frequency spectrum in a peak at a frequency, which depends on the distance and the relative velocity of the object in relation to the radar sensor.
Raindrops or splashing spray are in this sense objects which leave a weak peak in the frequency spectrum of the intermediate frequency signal at not too great distances from the radar sensor, usually at distances up to approximately 10 meters. In heavier precipitation, these peaks are added in the frequency range corresponding to the aforementioned distance range to yield a background signal, the so-called rain clutter. Due to the aforementioned distance range, the background of the intermediate frequency signal is elevated due to the rain clutter. At low vehicle speeds, the signal of the rain clutter is in the low frequency signal range of the intermediate frequency signal. At higher speeds, the signal of the rain clutter is shifted to higher frequencies in the intermediate frequency signal. Furthermore, the frequency position of the rain clutter depends on the slope of the frequency ramp of the emitted radar signal. The spectral power density in the frequency range of the rain clutter may serve as an indicator for the presence of precipitation. However, the relatively frequent case when reflection peaks from one or more actual objects are also within this frequency range, resulting in false detection of precipitation, is problematical. Such a situation occurs in particular when driving in alleys or tunnels, when driving directly next to a truck or when standing in a queue directly behind another vehicle.